1. Technical Field
This invention relates generally to integrated circuits and more particularly to drivers used in such integrated circuits.
2. Description of Related Art
As is known, an operational amplifier includes compensation to limit its bandwidth such that the operational amplifier is stable. Typically, the compensation is achieved by including a high precision capacitor. A high precision capacitor is required to ensure that its capacitance value does not vary significantly, which, if it did vary significantly, would cause the bandwidth of the operational amplifier to vary and potentially cause the operational amplifier to be unstable. As is further known, to achieve a high precision capacitor in a Complimentary Metal Oxide Semiconductor (CMOS) integrated circuit fabrication process, a metal capacitor is used. Such metal capacitors while providing accurate capacitance values consume relatively large amount of die area in comparison to the rest of the operational amplifier.
As is also known, in a CMOS integrated circuit fabrication process, a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) having its drain coupled to its source forms a small die area per capacitance value capacitor, which is typically called a MOS capacitor. However, as the gate voltage varies relative to the source/drain, the capacitance of a MOS capacitor varies substantially (e.g., by approximately 20%). Such a variation in capacitance is unacceptable for use to compensate an operational amplifier since the variation would cause the bandwidth of the amplifier to vary significantly. In particular, if the bandwidth gets too large when the capacitance is at a minimum, the operational amplifier can be unstable and cause performance degradation in the circuit.
Therefore, a need exists for an operational amplifier that utilizes the high capacitance density of a MOSFET while managing the capacitance variation of the MOSFET to maintain stability of the amplifier.